Optical disk

ABSTRACT

The present invention provides an optical disc which is constituted by a substrate, a recording layer, and a protection layer and is irradiated with a laser beam from the protection layer side. The optical disc has the substrate whose diameter is greater than 50 mm, the recording layer provided on the substrate, and the protection layer which is layered on the recording layer and thinner than the substrate and on which a recording or reproducing laser beam is incident. The total thickness of the optical disc is 0.7 mm or more.

TECHNICAL FIELD

[0001] The present invention relates to an optical disc and particularlyto an optical disc constituted by a substrate, a recording layer, and aprotection layer and irradiated with a laser beam from the side of theprotection layer.

[0002] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2002-232312 filed on Aug.9, 2002 in Japanese Patent Office, the entire contents of which areincorporated herein by reference.

BACKGROUND ART

[0003] In recent years, there have been demands for developments inrecording media for storing information recorded/processed, in view ofportable information recording/processing terminals, such as cellularphones, PDAs (Personal Digital Assistants), mobile personal computers,video cameras, digital still cameras, portable recording/reproducingdevices.

[0004] Developed as those recording media have been devices usingoptical discs as removable media which are cheap and easy to handle.Studies have further been being made on these devices.

[0005] Portable information recording/processing terminals have acompact size from the viewpoint of conveniences, so the optical discs tobe used must have a small size. However, the volume of data recordablein every disc must be large in consideration of, for example, use forvideo cameras and PDAs which deal with data of large informationamounts.

[0006] In order to realize recording or reproducing data at a highrecording density, there has been a popularly used method. In thismethod, the wavelength of a recording or reproducing laser beam has tobe shortened, and the numerical aperture (NA) of an objective lens whichconverges the recording or reproducing laser beam onto an optical dischas to be enlarged. Based on this method, discussions are made on astructure, for example, using blue light having a wavelength of 450 nmor less for the recording or reproducing laser beam, setting the NA ofthe objective lens to 0.78 or more, and an optical disc having a thinnerprotection layer than 0.1 mm.

[0007] If the optical disc having this thinner protection layer isapplied to an optical disc of a type having a small diameter of about 50mm, for example, data whose volume is as large as 2 GB or more can berecorded and reproduced per disc.

[0008] Meanwhile, optical discs used for portable informationrecording/processing terminals are exposed to extreme changes intemperature and humidity due to environmental changes as the discs arecarried. Hence, warping of the discs due to changes in temperature andhumidity must be small with respect to the optical discs used forportable information recording/processing terminals.

[0009] In case of the small diameter optical discs having a thinprotection layer as described above, detailed knowledge has not yet beenstocked about the warping according to changes in temperature andhumidity. There hence has been a conventional way of thinking that, forexample, the warping due to changes in temperature and/or humidity neednot be considered but the disc thickness may be decided fixing the ratiobetween the disc diameter and the disc thickness.

[0010] If the disc thickness is thus decided without considering changesin temperature or humidity, the optical discs are warped greatly due tochanges in temperature and humidity. This leads to a problem thataccurate recording/reproducing of information signals cannot beperformed on the optical discs.

DISCLOSURE OF THE INVENTION

[0011] An object of the present invention is to provide a novel opticaldisc capable of eliminating problems of the prior art as describedabove.

[0012] Another object of the present invention is to provide an opticaldisc capable of reducing warping due to change in temperature and/orhumidity, and a method of manufacturing the optical disc.

[0013] An optical disc according to the present invention comprises: asubstrate having a diameter greater than 50 mm; a recording layerprovided on the substrate; and a protection layer which is layered onthe recording layer and thinner than the substrate and on which arecording/reproducing laser beam is incident, wherein the optical dischas a total thickness of 0.7 mm or more.

[0014] Another optical disc according to the present inventioncomprises: a substrate having a diameter of about 80 mm; a recordinglayer provided on the substrate; and a protection layer which is layeredon the recording layer and having a thickness of 0.1 mm and on which arecording/reproducing laser beam is incident, wherein the optical dischas a total thickness of 0.94 mm or more.

[0015] Further another optical disc according to the present inventioncomprises: a substrate having a diameter of about 50 mm; a recordinglayer provided on the substrate; and a protection layer which is layeredon the recording layer and having a thickness of 0.1 mm and on which arecording/reproducing laser beam is incident, wherein the optical dischas a total thickness of 0.7 mm or more.

[0016] Further another optical disc according to the present inventioncomprises: a substrate having a diameter of about 51.5 mm; a recordinglayer provided on the substrate; and a protection layer which is layeredon the substrate and having a thickness of 0.08 mm and on which arecording/reproducing laser beam is incident, wherein the optical dischas a total thickness of 0.8 mm or more.

[0017] Further another optical disc according to the present inventioncomprises at least: a substrate; a recording layer layered on thesubstrate and having a single-layer structure or multilayer structure;and a protection layer which is layered on the recording layer andthinner than the substrate and on which a recording or reproducing laserbeam is incident, wherein the substrate and the recording layer areformed to satisfy conditions of expressions below: $\begin{matrix}{{{{\int_{0}^{n1}{{{E1}(y)}{e(y)}{y}}} + {\int_{n1}^{{n1} + {n2}}{{{E2}(y)}{e(y)}{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{{{Em}(y)}{e(y)}{y}}}} = {{\int_{0}^{n1}{{{\alpha 1}(y)}{{TE1}(y)}{y}}} + {\int_{n1}^{{n1} + {n2}}{{{\alpha 2}(y)}{{TE2}(y)}{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{\alpha \quad {m(y)}{{TEm}(y)}{y}}}}},} & (1) \\{{{{\int_{0}^{n1}{{{E1}(y)}{e(y)}y{y}}} + {\int_{n1}^{{n1} + {n2}}{{{E2}(y)}{e(y)}y{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{{{Em}(y)}{e(y)}y{y}}}} = {{\int_{0}^{n1}{{\alpha 1}\quad (y){{TE1}(y)}y{y}}} + {\int_{n1}^{{n1} + {n2}}{{{\alpha 2}(y)}{{TE2}(y)}y{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{\alpha \quad {m(y)}{{TEm}(y)}y\quad {y}}}}},} & (2)\end{matrix}$

 e(y)eo+ky  (3), and

θ=−kl  (4),

[0018] where y expresses displacement in a thickness direction of theoptical disc, ni (i=1, 2, 3, . . . , m) expresses a thickness of each ofthe layers forming the optical disc, m (m≧2) expresses a total number ofthe layers forming the optical disc, Ei (i=1, 2, 3, . . . , m) expressesa Young's modulus of each of the layers forming the optical disc, e(y)expresses a distortion generated in the optical disc, eo expresses aninitial distortion of the optical disc, αi (i=1, 2, 3, . . . , m)expresses an expansion coefficient depending on temperature and/orhumidity in each of the layers forming the optical disc, T expresses anamount of change in temperature and/or humidity around the optical disc,θ expresses a warping amount of the optical disc, k expresses acurvature of the optical disc, and 1 expresses a width of an area wherethe layers forming the optical disc are layered.

[0019] Further other objects and advantages achieved by the presentinvention will be more clearly understood from the embodiments describedbelow with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a cross-sectional view showing the schematic structureof an optical disc according to the present invention;

[0021]FIG. 2 is a graph showing a relationship between the disc diameterof an optical disc and the disc thickness of an optical disc obtained ina method of manufacturing an optical disc according to the presentinvention; and

[0022]FIG. 3 is a schematic view showing the structure of a device formeasuring the maximum warping amount at a predetermined disc thicknessof an optical disc.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] An optical disc according to the present invention will now bedescribed in detail with reference to the drawings.

[0024] In an optical disc 1, as shown in FIG. 1, informationrecording/reproducing layer 12 is formed on a substrate 11, and aprotection layer 13 is formed on the information recording/reproducinglayer 12. The information recording/reproducing layer 12 and theprotection layer 13 may each have a mono-layer structure or a multilayerstructure. The information recording/reproducing layer 12 has astructure in which one or more layers each including at least areflection film, a dielectric layer, and a recording layer are layered.Used as the recording film forming the information recording/reproducinglayer 12 can be a film made of a phase-change-type optical recordingmaterial, a magnetic optical recording film, or a recording filmcontaining organic pigments. The present invention uses, as therecording film, a film made of phase-change-type optical material.

[0025] The protection layer 13 provided in the optical disc 1 is 0.1 mmthick. The information recording/reproducing layer 12 is irradiated,through the protection layer 13, with a laser beam which has awavelength of 450 nm or less, e.g., 405 nm and converged by an objectivelens with an numerical aperture (NA) set to 0.78 or more, e.g., 0.8.

[0026] In the present invention, the thickness of each layer and thedisc thickness of the optical disc 1 which is the total thickness of thewhole disc are obtained by using a force-balancing expression and amoment-balancing expression among the layers forming the optical disc 1.The mentioned layers forming the optical disc 1 include, for example,those layers that form the multilayer structure in case where theinformation recording/reproducing layer 12 or the protection layer 13has a multilayer structure.

[0027] The force-balancing expression depending on the layers formingthe optical disc 1 is the following expression (5). $\begin{matrix}{{{\int_{0}^{n1}{{{E1}(y)}{e(y)}{y}}} + {\int_{n1}^{{n1} + {n2}}{{{E2}(y)}{e(y)}{y}}} + {\int_{{n1} + {n2}}^{{n1} + {n2} + {n3}}{{{E3}(y)}{e(y)}{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{{{Em}(y)}{e(y)}{y}}}} = {{\int_{0}^{n1}{{{\alpha 1}(y)}{{TE1}(y)}{y}}} + {\int_{n1}^{{n1} + {n2}}{{{\alpha 2}(y)}{{TE2}(y)}{y}}} + {\int_{{n1} + {n2}}^{{n1} + {n2} + {n3}}{{{\alpha 3}(y)}{{TE3}(y)}{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{\alpha \quad {m(y)}{{TEm}(y)}{y}}}}} & (5)\end{matrix}$

[0028] In this expression, y expresses displacement in a thicknessdirection of the optical disc 1, and ni (i=1, 2, 3, . . . , m) expressesa thickness of each of the layers forming the optical disc 1. Also, mexpresses the total number of the layers forming the optical disc 1, andEi (i=1, 2, 3, . . . , m) expresses a Young's modulus of each of thelayers forming the optical disc 1. Also, e(y) expresses a distortiongenerated in the optical disc 1, and αi (i=1, 2, 3, . . . , m) expressesan expansion coefficient depending on temperature and/or humidity ineach of the layers forming the optical disc. Further, T expresses anamount of change in temperature and/or humidity around the optical disc1.

[0029] The moment-balancing expression depending on the layers formingthe optical disc 1 is the following expression (6). $\begin{matrix}{{{\int_{0}^{n1}{{{E1}(y)}{e(y)}y{y}}} + {\int_{n1}^{{n1} + {n2}}{{{E2}(y)}{e(y)}y{y}}} + {\int_{{n1} + {n2}}^{{n1} + {n2} + {n3}}{{{E3}(y)}{e(y)}y{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{{{Em}(y)}{e(y)}y{y}}}} = {{\int_{0}^{n1}{{{\alpha 1}(y)}{{TE1}(y)}y{y}}} + {\int_{n1}^{{n1} + {n2}}{\alpha \quad 2(y){{TE2}(y)}y{y}}} + {\int_{{n1} + {n2}}^{{n1} + {n2} + {n3}}{{{\alpha 3}(y)}{{TE3}(y)}y{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{\alpha \quad {m(y)}{{TEm}(y)}y{y}}}}} & (6)\end{matrix}$

[0030] The distortion e(y) caused in the optical disc 1 is expressed bythe following expression (3) assuming circular warping.

e(y)=eo+κy  (3)

[0031] In this expression, eo expresses an initial distortion of theoptical disc 1, and κ expresses a curvature of the optical disc 1. Notethat the radius of curvature is 1/κ.

[0032] According to the above expressions (5), (6), and (3), thecurvature κ can be expressed by the following expression (7).$\begin{matrix}{K = \frac{{AQ} - {CP}}{{AQ} - {BC}}} & (7)\end{matrix}$

[0033] However, A, B, C, D, P, and Q in the expression (7) satisfy thefollowing expressions (8) and (9)

Ae0+Bκ=P  (8)

Ce0+Dκ=Q  (9)

[0034] On the other side, the relationship between the curvature and thewarping amount in the optical disc 1 is expressed by the followingexpression (4)

θ=−κl  (4)

[0035] In this expression, θ expresses a warping amount of the opticaldisc 1, and 1 expresses a width of an area where the layers forming theoptical disc 1 are layered, i.e., 1 is the length in radial direction ofthe area of the optical disc where signals are or can be recorded.

[0036] In optical discs, the disc diameter and the length in radialdirection of the area where signals are or can be recorded aresubstantially proportional to each other. Hence, the expression (4) canbe considered as showing the relationship between the diameter of theoptical disc and the warping amount of the optical disc.

[0037] To attain the thickness of each layer forming the optical disc 1and the thickness (of the whole disc) of the optical disc 1, forexample, the curvature κ of the optical disc 1 is firstly calculatedfrom the expression (4) using the length 1 in radius direction of thearea, which is proportional to the diameter of the optical disc 1 andrecords or can record signals, and the maximum tolerance of the warpingamount θ.

[0038] For example, if the optical disc 1 has a protection layer as thinas 0.1 mm, the maximum tolerance of the warping amount θ is 0.3 degrees.In this case, the warping amount of the optical disc is constrained to0.3 degrees or less because the warping amount of 0.3 degrees or lesscan reduce influence from aberration such as coma aberration.

[0039] From the curvature κ thus calculated and the expression (7), thethickness of each layer forming the optical disc 1 and the thickness ofthe substrate of the optical disc 1 can be obtained. For example, if 0.7mm is obtained as the thickness of the whole optical disc 1 on the basisof the above expressions, the thickness of the substrate 11 is obtainedto be 0.6 mm supposing that the protection layer 13 is 0.1 mm thick. Thethickness of the information recording/reproducing layer 12 issufficiently thin relative to the thicknesses of the substrate 11 andthe protection layer 13 hence need not be considered.

[0040] A substrate 11 having the thickness obtained in the above methodis prepared, for example, from polycarbonate, polyolefin, or the likewith use of an injection molding method or a press method. Aninformation recording/reproducing layer 12 having the thickness obtainedby the above method is formed as a film on the prepared substrate 11 bya sputtering method, a deposition method, a spin coating method, or thelike. A protection layer 13 having the thickness obtained by the abovemethod is formed on the formed information recording/reproducing layer12 from ultraviolet curing resin, polycarbonate, PMMA, amorphouspolyolefin, denaturalized acryl, or the like. The protection layer 13may alternatively formed by adhering a film forming the protection layer13 via ultraviolet curing resin, an adhesive agent, or an ultravioletcuring adhesive agent.

[0041] Thus in the method of manufacturing optical discs according tothe present invention, the expressions considering the warping amountdue to changes in temperature and/or humidity in correspondence with thediameter of the optical disc are used to decide the thickness of eachlayer forming the optical disc and the thickness of the whole disc.Therefore, no large warping is caused by environmental changes intemperature or humidity. Optical discs can be manufactured withcapability of accurately recording and/or reproducing informationsignals even in circumstances where environmental temperature orhumidity changes.

[0042]FIG. 2 shows the relationship between the disc diameter of theoptical disc and the thickness of the whole optical disc obtained by themethod described above. The optical disc whose whole thickness is thusobtained has the same structure as that of an optical disc which has thestructure shown in FIG. 1, a protection layer as thick as about 0.1 mmand a disc diameter of about 120 mm, except for the diameter of the discand the thickness of the whole disc.

[0043] The conditions are set as follows. That is, the maximum toleranceof the warping amount θ of the optical disc is set to 0.3 degrees. Thelength 1 in radial direction of the area of the optical disc wheresignals are recorded or can be recorded is set to about 60% of the totallength. The temperature varies by 50 degrees as an environmental change.

[0044] In FIG. 2, when the thickness of the whole disc is within thehatched area in the figure, the warping amount of the optical disc isrestricted to 0.3 degrees or less. This result can be used to decide thethickness of the whole disc which corresponds to the diameter of thedisc.

[0045] Table 1 shows the thickness of the whole disc which is necessaryto restrict the maximum warping amount to 0.3 degrees or less in casewhere the disc diameter is 50.5 mm, 80 mm, and 120 mm. It is found fromthe Table 1 that the thickness of the whole disc needs to be 0.7 mm ormore in case of a small-diameter disc which has the substrate 11, theinformation recording/reproducing layer 12, the protection layer asthick as 0.1 mm, and a disc diameter of 50.5 mm. The thickness of thesubstrate needs only to be 0.6 mm or more.

[0046] In case of a disc having a diameter of 80 mm, the thickness ofthe whole disc needs to be 0.94 mm or more. In this case, the thicknessof the substrate 11 needs only to be 0.84 mm or more which is the valueattained by subtracting 0.1 mm as the thickness of the protection layerfrom 0.94 mm as the thickness of the whole disc. TABLE 1 Disc diameter[mm] Disc thickness [mm] 50.5 0.7 80 0.94 120 1.18

[0047] Examples are not limited to the one described above. For example,the thickness of the entire optical disc 1 which satisfies the conditionof the maximum warping amount of 0.3 degrees or less is obtained to be0.8 mm from the expression (7), etc., like the above example, in casewhere the disc diameter of 51.5 mm and the thickness of the protectionlayer 13 is 0.08 mm. In this case, the thickness of the substrate can beobtained to be 0.72 mm.

[0048] In practical mass-production, the numerical values describedabove are used, adjusted in consideration of tolerances.

[0049]FIG. 3 shows a device which measures the maximum warping amountwith respect to a predetermined disc thickness of an optical disc. Awarping amount measuring device 20 has a laser light source 21, aposition-detecting-light detector 22, and a disc hold mechanism notshown. The disc hold mechanism includes a rotation mechanism not shownwhich rotates the optical disc 1 and a movement mechanism not shownwhich moves the rotation mechanism in the radial direction of theoptical disc.

[0050] In this warping amount measuring device 20, a laser beam emittedfrom the laser light source 21 is irradiated on the optical disc 1, andmeasures the position at which the reflected laser beam returns by useof the position-detecting-light detector 22. In accordance with thewarping amount of the optical disc 1, the position at which thereflected laser beam hits the position-detecting-light detector 22changes, so the warping amount can be measured by the change amount ofthis position. In this measurement, the in-circumference average ofwarping amounts measured by rotating once the optical disc 1 at apredetermined measurement radius is taken as the measured value at themeasurement radius. The position of the measurement radius is shifted inthe radial direction throughout the whole optical disc by the movementmechanism of the disc hold mechanism, the maximum measured value istaken as the maximum warping amount of the optical disc 1 having thepredetermined disc thickness.

[0051] Table 2 shows measurement results of maximum warping amountsmeasured at respective disc thicknesses by the warping amount measuringdevice 20 where the disc thickness of the optical disc, i.e., thethickness of the substrate is varied. The optical disc used in thismeasurement has a substrate layer, an information recording/reproducinglayer, and a polycarbonate protection layer having a thickness of 0.1mm, and is a small-diameter disc whose disc diameter is 50.5 mm. In thismeasurement, the temperature and humidity around the optical disc wererespectively set to 80 degrees and 85%, and the maximum warping amountswere measured after elapse of 96 hours. TABLE 2 Disc thickness [mm]Warping amount [degree] 0.8 0.23 0.7 0.26 0.6 0.36

[0052] As can be known from the table 2, the maximum warping amounts areas sufficiently small as 0.3 degrees or less when the disc thickness is0.7 mm or more in case of the small-diameter disc having a disc diameterof 50.5 mm.

[0053] Thus, the small-diameter disc having a disc diameter of 50.5 mmneeds only to have a disc thickness of 0.7 mm or more. The upper limitof the disc thickness, however, cannot be decided from the viewpoint ofthe warping amounts depending on changes in temperature and/or humidity.

[0054] Thus, in the optical disc according to the present invention, thedisc thickness of the small-diameter optical disc which has a diameterof about 50 mm is decided to be 0.7 mm or more by using the expressionsconsidering the warping amount depending on changes in temperatureand/or humidity. Therefore, no large warping is caused in the opticaldisc even when the temperature or humidity around the optical discchanges as the optical disc is used, for example, in a portableinformation recording/processing terminal and carried about.Accordingly, information signals can be recorded or reproducedaccurately.

[0055] In the above, the present invention has been describedexemplifying concrete structures. The present invention, however, is notlimited to the embodiments described above but can be modifiedvariously. For example, the present invention has been described withrespect to an optical disc having a structure as shown in FIG. 1. Thepresent invention is not limited hitherto but is applicable to anoptical disc having an arbitrary structure.

[0056] Although the present invention has been described with respect toan optical disc in which a substrate layer, an informationrecording/reproducing layer, and a protection layer are formed, thepresent invention is also applicable to an optical disc only having asubstrate layer and an information recording/reproducing layer. Further,although the present invention has been described with respect to anoptical disc which only has a substrate layer, an informationrecording/reproducing layer, and a protection layer formed, the layersforming the optical disc are not limited to these layers. For example,the present invention is also applicable to such an optical disc whichhas an intermediate layer between information recording/reproducinglayers forming a multilayer structure if the informationrecording/reproducing layer of the optical disc is made of themultilayer structure.

[0057] Further, in the present invention, it has been described that themaximum tolerance of the warping amount θ of the optical disc is, forexample, 0.3 degrees when the optical disc is made in a structure asshown in FIG. 1. However, if the optical disc 1 does not have thestructure as shown in FIG. 1, the maximum warping amount may be decidedsuch that influence from aberration can be restricted.

[0058] If an optical disc has a substrate, an informationrecording/reproducing layer, a protection layer having a thickness of0.1 mm, and a diameter of 80 mm and is characterized by having a discthickness of 0.94 mm or more, the same advantages can be achieved asthose of an optical disc having a diameter of 50 mm.

[0059] That is, the disc thickness of an optical disc having a smalldiameter of about 80 mm is decided to be 0.94 mm or more by usingexpressions considering warping amounts depending on changes intemperature and/or humidity. Therefore, no large warping is caused inthe optical disc even when the temperature or humidity around theoptical disc changes as the optical disc is used, for example, in aportable information recording/processing terminal and carried about.Accordingly, information signals can be recorded or reproducedaccurately.

[0060] The present invention is not limited to the above embodimentsdescribed with reference to the drawings. It would be apparent to thepersons in the art that various modifications, substitution, andequivalents thereof can be made without deviating from the scope andsubject matters of the claims included herein.

INDUSTRIAL APPLICABILITY

[0061] The optical disc according to the present invention is capable ofpreventing large warping even when temperature or humidity changesaround the disc and of accurately recording and reproducing informationsignals when applied to an optical disc having a small-diameter.

1. An optical disc comprising: a substrate having a diameter greaterthan 50 mm; a recording layer provided on the substrate; and aprotection layer which is layered on the recording layer and thinnerthan the substrate and on which a recording/reproducing laser beam isincident, wherein the optical disc has a total thickness of 0.7 mm ormore.
 2. The optical disc according to claim 1, wherein the substratehas a thickness of 0.6 mm or more.
 3. The optical disc according toclaim 1, wherein the protection layer has a thickness of about 0.1 mm.4. An optical disc comprising: a substrate having a diameter of about 80mm; a recording layer provided on the substrate; and a protection layerwhich is layered on the recording layer and having a thickness of 0.1 mmand on which a recording/reproducing laser beam is incident, wherein theoptical disc has a total thickness of 0.94 mm or more.
 5. The opticaldisc according to claim 4, wherein the substrate has a thickness of 0.84mm or more.
 6. An optical disc comprising: a substrate having a diameterof about 50 mm; a recording layer provided on the substrate; and aprotection layer which is layered on the recording layer and having athickness of 0.1 mm and on which a recording/reproducing laser beam isincident, wherein the optical disc has a total thickness of 0.7 mm ormore.
 7. The optical disc according to claim 6, wherein the substratehas a thickness of 0.6 mm or more.
 8. An optical disc comprising: asubstrate having a diameter of about 51.5 mm; a recording layer providedon the substrate; and a protection layer which is layered on thesubstrate and having a thickness of 0.08 mm and on which arecording/reproducing laser beam is incident, wherein the optical dischas a total thickness of 0.8 mm or more.
 9. The optical disc accordingto claim 8, wherein the substrate has a thickness of 0.72 mm or more.10. An optical disc comprising at least: a substrate; a recording layerlayered on the substrate and having a single-layer structure ormultilayer structure; and a protection layer which is layered on therecording layer and thinner than the substrate and on which a recordingor reproducing laser beam is incident, wherein the substrate and therecording layer are formed to satisfy conditions of expressions below:$\begin{matrix}{{{{\int_{0}^{n1}{{{E1}(y)}{e(y)}{y}}} + {\int_{n1}^{{n1} + {n2}}{{{E2}(y)}{e(y)}{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{{{Em}(y)}{e(y)}{y}}}} = {{\int_{0}^{n1}{{{\alpha 1}(y)}{{TE1}(y)}{y}}} + {\int_{n1}^{{n1} + {n2}}{{{\alpha 2}(y)}{{TE2}(y)}{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{\alpha \quad {m(y)}{{TEm}(y)}{y}}}}},} & (1) \\{{{{\int_{0}^{n1}{{{E1}(y)}{e(y)}y{y}}} + {\int_{n1}^{{n1} + {n2}}{{{E2}(y)}{e(y)}y{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{{{Em}(y)}{e(y)}y{y}}}} = {{\int_{0}^{n1}{{\alpha 1}\quad (y){{TE1}(y)}y{y}}} + {\int_{n1}^{{n1} + {n2}}{{{\alpha 2}(y)}{{TE2}(y)}y{y}}} + \ldots + {\int_{{n1} + \ldots + {n\quad m} - 1}^{{n1} + \ldots + {n\quad m}}{\alpha \quad {m(y)}{{TEm}(y)}y\quad {y}}}}},} & (2)\end{matrix}$

e(y)=eo+ky  (3), andθ=−kl  (4), where y expresses displacement in athickness direction of the optical disc, ni (i=1, 2, 3, . . . , m)expresses a thickness of each of the layers forming the optical disc, m(m≧2) expresses a total number of the layers forming the optical disc,Ei (i=1, 2, 3, . . . , m) expresses a Young's modulus of each of thelayers forming the optical disc, e(y) expresses a distortion generatedin the optical disc, eo expresses an initial distortion of the opticaldisc, αi (i=1, 2, 3, . . . , m) expresses an expansion coefficientdepending on temperature and/or humidity in each of the layers formingthe optical disc, T expresses an amount of change in temperature and/orhumidity around the optical disc, θ expresses a warping amount of theoptical disc, k expresses a curvature of the optical disc, and lexpresses a width of an area where the layers forming the optical discare layered.
 11. The optical disc according to claim 10, wherein thecurvature k of the optical disc is calculated from the maximum warpingamount θ tolerable for the optical disc, the width l of the area wherethe layers forming the optical disc are layered, and the expression (4),the thickness of each of the layers forming the optical disc is obtainedby using the calculated curvature k of the optical disc and theexpressions (1) to (3).
 12. The optical disc according to claim 11,wherein the optical disc has a diameter of 80 mm or less.
 13. Theoptical disc according to claim 12, wherein the protection layer has athickness of 0.1 mm or less.